The in-situ extraction of minerals often involves the application of heat to enhance viscosity reduction, partial decomposition and upgrading, and solubility. Examples of fossil fuels subject to in-situ extraction are oil sands, oil shale, and coal. In some cases, the uniformity of heat application is desirable, because too little heat may reduce the extent of the desired changes facilitating extraction, and too much heat may degrade the desired products into less valuable products. For example, an effective heat transfer to surrounding mineral deposits promotes enough in-situ cracking and hydrogenation for oil sands and oil shale recovery to provide a premium quality synthetic crude oil without cracking a substantial portion to less valuable gas and the formation of coke.
Effective extraction of valuable products from of these types of mineral formations involves distribution of heat throughout a large volume of ore. Consequently, applying the heat at the highest possible temperature is desirable. However, heterogeneities in geology may affect the rate at which the formation may accept and dissipate heat. If the power of the heater is constant along its length, that may cause underheating or overheating in parts of the formation that dissipate the heat more quickly or more slowly than average. That underheating or overheating might cause local underconversion or product degradation.
Some existing systems intended to prevent overheating involve temperature-limited electric heaters that are designed for in-situ mineral extraction. The temperature limit enables the maximum allowable power to be applied to the entire formation, even when the heat acceptance varies with location in the formation. The resistance of the heating elements or dielectrics in the heaters is often temperature dependent, such that the power lowers as a target temperature is reached to prevent overheating. Such methods may, for example, use the Curie point of the conductor to change its resistance at a desired maximum temperature.